Another big difference is that the 4 has a freewheel hub while the 5 has a freehub. Freewheels leave a long unsupported axle length on the drive side which often results in bent and broken axles when bikes are ridden hard. Freehubs have the bearings positioned closer to the end of the axle. If your kid is small and not into jumping and whatnot than this might not make a big difference.